Method of operating thermionic tubes and cathode therefor



C. V. LITTON Jan. 8, 1946.

METHOD OF OPERATING THERMIONIC TUBE AND CATHODE THEREFOR Filed March 20, 1944' INVENTOR CHARLES l/ l/rro/v.

ATTORNEY Patented Jena, 1946 METHOD OF OPERATING THERMION IC TUBES AND CATHODE THEREFOR Charles v. Litton, Redwood City, Calii'., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application March 20, 1944, Serial No. 527,208

12 Claims.

I a cathode construction fora thermionic tube in which modulation effects caused by heating the cathode by alternating current are eliminated.

Another object of the invention is to provide a unipotentlal cathode for a thermionic tube which is heated by bombardment from an electron emitting element adjacent the cathode.

,Another object of the invention is to provide a method of operating a thermionic tube whereby modulation eflects, caused by heating the cathode by alternating current, are eliminated.

Other objects and objects relating to the method of constructing and assembling. the various parts of the cathode will be apparent as the description of the invention proceeds.

The invention has been illustrated in the accompanying drawing in which Fig. 1 is a sectional side elevational view of a portion of a thermionic tu-be structure showing one form of the cathode of the invention; and

Fig. 2 is an enlarged fragmentary sectional view of the wall of the cathode of Fig. 1.

According to the invention a cathode is heated by means of electron bombardment from an electron emitting element which is initially heated to the proper temperature by passing an alternating current through it. The novel construction of the cathode permits sufficient heat to be radiated from the cathode to the electron emitting element to maintain the latter at the proper temperature, whereupon the alternating current may be shut off and the cathode will continue to operate.

In Fig. 1 a portion of a high power thermionic tube is shown having an upper envelope I of glass or other suitable insulating material fused in a known manner to a disc 2, made of some suitable metal, to the underside of which is fused a lower envelope 3. The disc 2 forms a support and a terminal for the tubular cathode 4 which is mounted upon the end of a tapered tubular member 5 of thin metal having a flange 5 at its base by means of which it is attached to the disc 2, the attachment being indicated as rivets 6 which extends through the flange and through the disc 2.

scribed later. Around the sleeve 1 is wrapped a rod In which may be of tungsten and is preferably about .125 inch in diameter. This rod is wound closely around the sleeve 1 with the turns touching each other and is preferably continuously welded to the sleeve 1, as indicated 'at H in Fig. 2, to provide good heat conductivity between the sleeve and the turns of the rod. Outside of the rod I0 I provide an electron emitting coating which may preferably be formed of a thoriated tungsten wire. or filament l2 wound around the entire cathode structure, as indicated more clearly in Fig. 2. This wire l2 may be about .020 inch in diameter.

The upper end of the cathode sleeve I is provided with a closure member l3 which may be suitably attached to the sleeve by means, for instance, of a ring member I4 having a U-shaped cross-section. The closure member 13 prevents heat from escaping from inside of the cathode structure, so that the cathode will act as an oven to maintain a high temperature within it.

An electron emitting element I5 is provided within the cathode 4, extending longitudinally thereof. This element may comprise a double helix l6 of wire which is attached at the upper ends of the helices to a, rod ll extending up from the disc 2 upon which it is supported. The rod- I1 is, however, insulated from the disc 2 by means of a suitable insulator l8 and may form one terminal of the electron emitting element. The lower end of the helices l6 may also be connected to the rod I! at a point adjacent the lower end of the sleeve 1, so that both ends of both wires are connected to the rod IT. A pair of rods I9 and 20 are also mounted upon the disc 2 from which they areinsulated by suitable insulators 2| and 22, respectively, and these rods extend up inside of the helices I6 to a, point adjacent the center thereof. and have their ends attached, one to one helix and one to the other. The arrangement provides a connection for the two halves of one helix in parallel between the rod is and the rod I! and forthe two halves of the other helix in parallel between the rod 20 and the rod ll. The electron emitting element is thus in effect four separate wire coils energized'by voltage applied between the rod ll as one terminal and the rods [9 and 20, connected together, as the other terminal. The wire I6 is made electron emitting in accordance with the invention inany desired manner. For instance, it may be made of thoriated tungsten wire.

In operating the tube of the invention a potential difierence is applied between the cathode 4 and the heating element 15, as indicated by the battery 23, which is suflicient and of the proper polarity to cause electrons to leave the element l5 and bombard the inner walls of the cathode when the temperature of the element I 5 is raised to electron emitting temperature. An alternatthe rods l9 and 20 which causes the wires I6 to rise in temperature and emit electrons to the inner wall of the sleeve I. The bombardment of electrons upon the inner wall of the sleeve 1 causes the temperature of the cathode structure to rise and to continue to rise until the temperature is sufiicient to cause the electron emitting coating l2 on the outside of the turns of the rod III to emit suflicient electrons for the operation.

of the tube. Thereupon, the alternating voltage across the electron emitting element l5 may be cut oil, but this element will remain at sumcient temperature to continue to emit electrons by virtue of its position within the heated cathode.

The cathode is then continuously heated by bombardment from the electron emitting element which in turn is maintained at the proper temperature by heat radiating from the oathode structure. Since there is no longer any alternating current flowing in the electron emitting element l5, no alternating magnetic or electrostatic fields are set up, and therefore there is no modulating effect on the electrons emitted from the cathode. The output is thus free from this type of distortion.

In order to prevent undue cooling of the cathode structure at the ends and so maintain a uniform electron emission from end to end of the cathode structure, I increase the potential gradient between the cathode and the heating element at the ends of the cathode by turning in the ends at 8 and 9, as already mentioned.

It will be seen from the above description that I have provided a cathode structure for a thermionic tube the operation of which may be initiated by alternating current but which eliminates the disadvantage of a cathode heated by alternating current by permitting the supply of alternating current to be cut of? as soon as the cathode has been raised to the proper temperature. I have also provided a method of operating a thermionic tube whereby these same ad- I vantageous results are achieved.

What I desire to claim and secure by Letters Patent is:

1. A thermionic tube comprising a tubular cathode, an electron emitting element extending longitudinally within said cathode and spaced from the wall thereof, means to apply a potential difference between said element and said cathode sufiicient to cause bombardment of the inner surface of said cathode by electrons when said element is at electron emitting temperature, means for applying current to said element to heat it to electron emitting temperature, the relation of said cathode to said element being such that the cathode acts as an oven to heat said element to operating electron emitting temperature whereby the current supply may be disconnected from said first element after said cathode has once been brought up to electron emitting temperature and said element will thereafter be maintained at electron emitting temperature, thus maintaining the operating temperature of said cathode.

2. A thermionic tube as defined in claim 1 in.

- ing current is then applied between the rod l1 and the center, whereby the potential gradient between said element and said cathode is greater near the ends of said cathode.

3. A cathode structure comprising a thin metal sleeve,- a metal rod wound around said sleeve in close contact therewith, an electron emitting element within said sleeve extending longitudinally thereof and-spaced from the inner walls, means to apply heating current to said electron emitting element to heat it to electron emitting temperature means to apply a potential between said cathode and said element to cause electron bombardment thereof and means to retain a sufporting rod concentrically mounted within said sleeve, a helical electron emitting wire having its ends connected to said rod and its intermediate portion separated in spaced relation from said rod, and an electrical connection to the midpoint of said wire.

5. A cathode structure for thermionic tube comprising a thin metal sleeve, a metal. rod wrapped around said sleeve and in close contact therewith, an electron emitting filament wrapped around the outside of the turns of said rod so as to produce an electron emitting outer surface for said sleeve, an electron emitting element mounted within said sleeve and extending longitudinally thereof spaced from the walls thereof, means to energize said electron emitting element with alternating current,'and means to maintain a potential difierence between said electron emitting element and said sleeve, so as to cause bombardment of said sleeve by electrons from said element when said element is heated to electron emitting temperature.

6. A cathode structure as specified in claim 12 in which the cathode sleeve is made of tantalum and the metal rod of tungsten.

7. The method of operating a thermionic tube having an exteriorly emissive cathode sleeve sur-- rounding andin spaced relation to an electron emitter which comprises heating the electron emitter to electron emitting temperature by an electric current, bombarding the cathode sleeve of said tube with electrons from said electron emitter to raise the temperature of said cathode sleeve to electron emitting temperature, transferring sufficient heat from said cathode sleeve to said electron emitter to maintain said electron emitter at electron emitting temperature, and thereafter shutting off said electric current.

8. In a thermionic tube having a cathode in good heat transfer relation with an electron emitter, the method of maintaining the temperature of the cathode of the thermionic tube at electron emitting temperature which comprises transferring sufllcient heat from said cathode to said electron emitter to maintain said electron emitter at electron emitting temperature :and

bombarding said cathode with electrons from said electron emitter.

9. A thermionic tube comprising a hollow cathode with an emissive outer surface and an electron emitting element within said ,cathode the relation of said cathode and element being such that when said element is heated to emission temperature by application of a heating current thereto and a potential is applied between said cathode and said element to produce electron bombardment of said cathode suificient to heat it to electron emissive temperature the cathode acts as an oven to maintain said element at an operative electron emissive temperature even in the absence of said heating current, whereby the latter may be removed during the operation of the tube.

10. A device in accordance with claim 9 wherein said electron emissive outer surface and said element consists of thoriated tungsten wire.

11. A cathode structure comprising a tubular cathode and an electron emitting element and coaxial with said cathode and in spaced relation with respect thereto, the distance between said cathode and said element being less opposite the ends of said element than at the center thereof whereby, when a potential is applied between said cathode and said element to cause electrons from the latter to bombard the former, the potential radient between said element and said cathode is greater near the ends of said cathode.

12. A cathode structure for a thermionic tube comprising a thin metal sleeve, a metal rod wrapped around said sleeve and in close contact therewith and an electron emissive wire wound around the outside of said rod.

CHARLES V. LIT'I'ON. 

